FIELD OF THE INVENTION
The present invention relates to a device for measuring pupillary distance (referred to also as PD value hereinafter) of a subject to obtain data necessary for optimumly setting a lens in a spectacle frame.
Setting of a lens in spectacles frame essentially requires that the visual axis of the subject at a viewing distance under an average state of use coincides with the optical axis of the lens. In particular, a high degree of coincidence is required in the case of a progressive lens which has recently been developed and put into use. In general, a progressive lens comprises three portions of different optical characteristics: namely, a farvision portion which is used when sighting an object which is at a long distance from the subject, a near-vision portion which is used when sighting an object which is at a short distance from the subject, and a progressive portion in which a progressive zone is an optically usable region. When sighting condition with this lens is changed from a far-vision state to a near-vision state, it is necessary that the visual axis is progressively moved from the far-vision portion to the near-vision portion, across the narrow progressive zone.
Any discrepancy between the visual axis of the subject and the optical axis of the lens causes problems such as generation of prism or failure in obtaining expected correction effect.
A typical known pupillary distance measuring device has the following construction. When the subject is made to sight at a spot light source (a fixation target), a cornea reflection image (bright spot) is formed. The position of this bright spot as viewed from the same side as the light source is defined as a vertex of cornea. The position of the vertex of cornea is regarded as the position passed by the visual axis. The distance between the vertex of the cornea and the nose of the subject is measured as the pupillary distance. The viewing distance is variable by shifting a convexed lens.
This known arrangement is disclosed in detail in the specification of the U.S. Pat. No. 3,495,897. The shifting of the convexed lens requires a linear motion mechanism including various parts such as a viewing distance setting knob, a slide guide interlocked with the knob, a connecting rod and so forth, with the result that the device as a whole is highly complicated in construction.